Method and system for embedding message data in a digital image sequence

ABSTRACT

A method for embedding message data in a digital image sequence having two or more frames, includes the steps of: providing a dispersed message image representative of the message data; and adding spatially shifted versions of the dispersed message image to successive frames of the digital image sequence.

FIELD OF THE INVENTION

The invention relates generally to the field of digital imageprocessing, and in particular to a method for embedding watermarks indigital image sequences.

BACKGROUND OF THE INVENTION

Digital watermarking refers to the embedding of a hidden message in animage or image sequence for such purposes as establishing ownership,tracking the origin of the data, preventing unauthorized copying, orconveying additional information (meta-data) about the content.Watermarking has potential uses in a wide range of products, includingdigital still and video cameras, printers and other hardcopy outputdevices, and content delivery services (e.g., Internet-basedphotofinishing). Recently, there has been significant interest in theelectronic distribution and display of theatrical movies, which istermed digital cinema. Studios and distributors have a strong need toprotect the movie content from unauthorized use, and watermarking canassist by establishing ownership and tracing the source of stolencontent (through the use of hidden date/time/location stamps inserted atthe time of the movie distribution and/or presentation). The presentinvention relates specifically to the watermarking of image sequences,and thus it has usefulness in an application such as digital cinema.

Numerous watermarking methods have been described in the prior art,including both patents and the technical literature. Many of thesemethods are described in review papers such as: Hartung and Kutter,Multimedia Watermarking Techniques,” Proc. IEEE, 87(7), pp. 1079-1107(1999), and Wolfgang et al., Perceptual Watermarks for Digital Imagesand Video, Proc. IEEE, 87(7), pp. 1108-1126 (1999).

A basic distinction between various methods is whether the watermark isapplied in the spatial domain or the frequency domain. In eitherapproach, it is common for a pseudo-random (PN) sequence to be used inthe watermark generation and extraction processes. The PN sequenceserves as a carrier signal, which is modulated by the original messagedata, resulting in dispersed message data (i.e. the watermark) that isdistributed across a number of pixels in the image. A secret key (i.e.seed value) is commonly used in generating the PN sequence, andknowledge of the key is required to extract the watermark and theassociated original message data.

As noted in the review papers by Hartung et al. and by Wolfgang et al.,most research on watermarking techniques has focused on single-frameimages, and there are significantly fewer methods that are specific toimage sequences (i.e. video watermarking). Of course, a watermarkingmethod that has been designed for single-frame images could be appliedto an image sequence by merely repeating the same process for eachframe. However, this approach has the disadvantage that the fixedwatermark pattern may become perceptually objectionable when the imagesequence is displayed in real-time.

There are several prior art patents that include video-specificwatermarking methods: U.S. Pat. No. 5,809,139 issued Sep. 15, 1998 toGirod et al. entitled Watermarking Method and Apparatus for CompressedDigital Video; U.S. Pat. No. 5,901,178 issued May 4, 1999 to Lee et al.entitled Post-Compression Hidden Data Transport for Video; U.S. Pat. No.5,991,426, issued Nov. 23, 1999 to Cox et al. entitled Field-BasedWatermark Insertion and Detection; U.S. Pat. No. 6,026,193 issued Feb.15, 2000 to Rhoads entitled Video Steganography.

In the patents by Girod et al. and Lee et al., the methods are designedfor directly embedding a watermark in compressed frequency-domain videostreams (such as MPEG-encoded sequences). The patent by Cox et al.describes a method for alternately embedding positive and negativewatermarks in consecutive fields of an interlaced video signal; thismethod is not suitable for progressively scanned image sequences such asthose used in digital cinema applications. The patent by Rhoadsdiscloses the basic concept of using multiple watermarked frames from animage sequence to extract the watermark with a higher degree ofconfidence than would be obtained with only a single frame. However, themethods described in all of the aforementioned patents make use of thesame watermarking pattern in each successive frame of the sequence. As aresult, these methods are subject to the same disadvantage as previouslymentioned, namely, the presence of a fixed watermark pattern that can beobjectionable.

There are obvious modifications that can eliminate the fixed watermarkpattern, but they also suffer from limitations. One modification is tochange the PN carrier from frame to frame, but this may necessitate abrute-force search of all possible carriers when performing thewatermark extraction process. The management of the secret keys that areused in generating the PN sequences also becomes problematic. Anothermodification is to change the message while using the same carrier, butit may not be desirable to change the message from frame to frame inmany applications. Moreover, either modification does not allowinformation from multiple frames to be directly combined when extractingthe watermark. This limitation reduces the robustness of the watermarkextraction process to certain types of removal attacks.

There is a need therefore to have an image sequence watermarkingtechnique that: (1) minimizes the visibility of the watermark when thewatermarked sequence is displayed in real-time, (2) requires only asingle key for the generation and extraction of the watermark data, and(3) allows for information from multiple frames to be combined whenextracting the watermark.

SUMMARY OF THE INVENTION

The need is met according to the present invention by providing a methodfor embedding message data in a digital image sequence having two ormore frames, that includes the steps of: providing a dispersed messageimage representative of the message data; and combining spatiallyshifted versions of the dispersed message image with successive framesof the digital image sequence.

ADVANTAGES

The present invention minimizes the visibility of a watermark in animage sequence while simultaneously providing the convenience of asingle-key system. The invention also allows watermark information to becombined from multiple frames, which improves the robustness of thewatermark extraction process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a prior art method forembedding a watermark in an original image;

FIG. 2 is a schematic diagram illustrating a prior art method forextracting a watermark from an image containing an embedded watermark;

FIG. 3 is a schematic diagram illustrating the spatial shifting of thedispersed message image between frames in the present invention;

FIG. 4 is an example of the potential misalignment between embeddedtiles and an extracted tile during the watermark extraction process;

FIG. 5 illustrates the effect of tile misalignment on an extractedmessage; and

FIG. 6 is a block diagram of a method for determining the tile offsetusing a message template.

DETAILED DESCRIPTION OF THE INVENTION

The present invention overcomes the limitations of the prior art byusing a single carrier image (and hence provides the convenience of asingle key) to generate a dispersed message image, but the dispersedmessage image is spatially shifted from frame to frame. The shifting maybe done using either a deterministic or random offset between frames.The shifting process minimizes the visibility of a watermark bypreventing spatial alignment of the watermark pattern from frame toframe. The shifting does not substantially degrade the robustness of thewatermark extraction process when it is applied to a single frame,because the shifting acts in a similar manner as cropping of the image.Most watermark techniques are designed to be robust to cropping since itis a common image processing operation. Moreover, because the samecarrier image is used for each frame, the extraction process can easilycombine information from multiple frames (after suitable alignment) toprovide more robust extraction of the watermark. The present inventionis aimed primarily at watermark methods that embed in the spatialdomain. However, it can also be applied to some frequency domain methodsthat use local frequency decompositions, e.g., block-basedtransformations.

The present invention is preferably implemented by a programmed digitalcomputer. The computer can be a general purpose digital computer or aspecial purpose computer for digital image processing. It is within theordinary skill in the programming art to provide a computer program forpracticing the present invention from the following description of theinvention.

A preferred data embedding technique for use with the present inventionis disclosed in U.S. Pat. No. 6,044,156 issued Mar. 28, 2000 toHonsinger et al. entitled Method for Generating an Improved Carrier forUse in an Image Data Embedding Application. This patent is included inits entirety by reference. Referring to FIG. 1, in this technique, anoriginal two-dimensional image 10, I(x, y), is processed to produce awatermarked image 12, I′(x, y). A two-dimensional message 14, M(x, y),represents the data to be embedded in the original image 10. In its mostgeneral form, the message 14 is an image, and it can represent an icon16 (e.g. a trademark), or it can represent the bits 18 in a binarymessage. In the latter case, the on and off states of the bits arerepresented as plus and minus ones (more specifically, positive andnegative delta functions), which are placed in predefined and uniquelocations across the message image. Examples of iconic message data aretrademarks, corporate logos or other arbitrary images. Performancegenerally decreases as the message energy increases so edge maps of theicons are used. Examples of binary message data are 32-bitrepresentations of URL's, and copyright ID codes, or authenticationinformation.

As shown in FIG. 1, the fundamental steps for embedding message data inan original image with this method are:

-   1. A n×n message image 14, M(x, y), is generated from the message    data;-   2. The message image 14 is circularly convolved 20 with a n×n    carrier image 22, C(x, y), to produce a n×n dispersed message image    24. The carrier image may be produced using a secure key 26 as is    known in the prior art;-   3. The dispersed message image 24 is scaled 28 in amplitude using a    multiplicative factor α; and-   4. The scaled dispersed message image 30 is added to the original    image 10 as contiguous n×n tiles to form a watermarked image 12,    I′(x, y).

The tiling of the dispersed message image forms the watermark patternthat is combined with the original image. The scaling factor α is anarbitrary constant chosen to make the watermark pattern simultaneouslyinvisible and robust to common processing. Typically, the size of thedispersed message image 24 is chosen to be smaller than the size oforiginal image 10, and the tiling process allows the scaled dispersedmessage 30 to be repetitively embedded over the extent of the originalimage 10. The repetitive structure provides robustness to the watermarkwhen image processing operations (such as cropping, compression, lowpassfiltering, etc.) are applied to the watermarked image. Otherwatermarking techniques use different methods for embedding the messagedata, but the repetitive nature of the embedding process is a commonaspect because of this improved robustness.

This embedding process for each tile can be described mathematically as:I′(x,y)=α[M(x,y)*C(x,y)]+I(x,y),  (1)where the symbol * represents circular convolution. From Fourier theory,spatial convolution is equivalent in the frequency domain to addingphase while multiplying magnitudes. Therefore, the effect of convolvingthe message image 14 with the carrier image 22 is to distribute themessage energy in accordance with the phase of the carrier image and tomodulate the amplitude spectrum of the message image with the amplitudespectrum of the carrier image. If the message image were a single deltafunction, δ(x, y), and the carrier image had random phase andsubstantially flat Fourier magnitude, the effect of convolving with thecarrier image would be to distribute the delta function over space.Similarly, the effect of convolving a message image with a random phasecarrier image is to spatially disperse the message energy.

As shown in FIG. 2, the process as described by Honsinger et al. forextracting the message data from a watermarked image consists of thefollowing fundamental steps:

-   1. Contiguous n×n tiles 12′ are formed from the watermarked image    12, I′(x, y);-   2. The tiles 12′ are averaged 32 across each spatial location (x, y)    to form an averaged tile 34;-   3. The averaged tile 34 is circularly correlated 36 with the n×n    carrier image 22, C(x,y), to produce an extracted n×n message image    14′, M′(x,y); and-   4. The message data is recovered from the extracted message image    14′.

The averaging 32 of the individual tiles 12′ produces a better estimateof the message data (i.e., it improves the signal-to-noise ratio)because the dispersed message image in each tile will add constructively(since it is the same in each tile), while the corresponding originalimage content in each tile will add destructively (since it is typicallydifferent in each tile).

This watermark extraction process can be described mathematically as:$\begin{matrix}\begin{matrix}{{M^{\prime}( {x,y} )} = {{I^{\prime}( {x,y} )} \otimes {C( {x,y} )}}} \\{= {{{\alpha\lbrack {{M( {x,y} )}*{C( {x,y} )}} \rbrack} \otimes {C( {x,y} )}} + {{I( {x,y} )} \otimes {C( {x,y} )}}}}\end{matrix} & (2)\end{matrix}$where the symbol, {circle around (×)}, represents circular correlation.Correlation is similar to convolution in that Fourier magnitudes alsomultiply. In correlation, however, phase subtracts. Therefore, the phaseof the carrier image subtracts when the watermarked image is correlatedwith the carrier image, thus leaving the message image. Indeed, if weagain assume that the carrier image is designed to have a substantiallyflat Fourier amplitude, then the process of correlation of the carrierimage on the watermarked image Eq. 2, can be reduced to:M′(x,y)=αM(x,y)+noise.  (3)

That is, the extracted message image is a scaled version of the originalmessage image plus noise due to the cross correlation of the originalimage with the carrier image.

More generally, we can rewrite Eq. 2 as:M′(x,y)=αM(x,y)*[C(x,y){circle around (×)}C(x,y)]+noise.  (4)

The above equation suggests that the resolution of the extracted messageimage is fundamentally limited by the autocorrelation function of thecarrier image, C(x,y){circle around (×)}C(x,y). Any broadening ofC(x,y){circle around (×)}C(x,y) from a delta function will blur theextracted message image when compared to the original message image.Another way to view the effect of the carrier image on the extractedmessage image is to consider C(x,y){circle around (×)}C(x,y) as a pointspread function, since convolution of the original message image withC(x,y){circle around (×)}C(x,y) largely determines the extracted messageimage.

In a typical application of this watermarking process, the tiling of thedispersed message image is performed using the same tile locations foreach original image. Typically, the tiles would be arranged by startingwith a full tile in the upper left corner of the image, and then placingadditional tiles as needed to cover the original image. If the originalimage size is not an integer multiple of the tile size, there will beborder regions that do not contain full tiles. These regions can beignored during the extraction process.

As described previously, the typical application of this watermarkingprocess to an image sequence results in a fixed watermark pattern foreach frame. This fixed pattern may be objectionable when the sequence isviewed. The present invention overcomes this limitation by spatiallyshifting the tile locations (and hence the watermark pattern) from frameto frame. While the tiles are still placed in a contiguous manner withina frame, the first tile in the frame is shifted by an integer number ofpixels relative to the first tile in the previous frame. This process isshown in FIG. 3 for the three consecutive frames. The shifting processis cyclical, i.e., the tile pattern can be viewed as connected cylindersin the horizontal and vertical directions. In this way, the watermarkpattern always covers the original image regardless of the amount of theshift.

For the present invention to work effectively, the extraction processmust be able to recover the embedded message image even when thewatermark pattern has been shifted from its nominal position. Duringextraction, n×n tiles are formed from the watermarked image, but thereis no guarantee that these extracted tiles will be aligned with theoriginal watermark tile boundaries. This situation is illustrated inFIG. 4. This is known as the synchronization problem, and it is the sameproblem that occurs when a watermarked image has been cropped by anunknown amount. In the following, we describe how the preferredembodiment can synchronize a watermark pattern that has been shifted byan unknown amount.

The ability to recover from cropping is an essential component of awatermarking algorithm. In the preferred embodiment, if an arbitrarilylocated n×n region is extracted from a watermarked image, the extractedmessage image from this region would probably appear to be circularlyshifted since it is unlikely that the extraction occurred along theoriginal tile boundary. Indeed, if the origin of the n×n extractedregion is a distance, (Δx,Δy), from its nearest original tile boundary,then the extracted message image will be circularly shifted by (Δx,Δy),i.e. M′(x−Δx,y−Δy). This effect of this circular shift on the extractedmessage image is shown in FIG. 5.

On the surface, this circular shift ambiguity is a severe limitation ondata capacity because it would appear that the message structure must beinvariant to cyclic shifts. However, it is also possible to determine(Δx,Δy) under certain conditions, and thus realign the extracted messageimage. As described in copending application, U.S. Ser. No. 09/453,160filed Dec. 2, 1999 by Honsinger, this can be accomplished by placing thebits in the message image in a special manner. Specifically, a messagetemplate is used, which is a prescription of where to place the bits inthe message image. The message template, T(x,y), is derived by placingpositive delta functions on a blank n×n image such that each deltafunction is located a minimum distance away from all others and suchthat the autocorrelation of the message image is as close as possible toa delta function. In other words, the bits are placed such that themessage template autocorrelation sidelobes have minimal amplitude.

Now, correlation of the extracted tile with a zero mean carrier imageguarantees that the circularly shifted extracted message imageM′(x−Δx,y−Δy) is also zero mean. As a result, the absolute value of theextracted message image must be practically equivalent to a circularlyshifted message template. That is|M′(x−Δx,y−Δy)|=T(x,y)*δ(x−Δx,y−Δy).  (5)

As shown in FIG. 6, due to the autocorrelation property of the messagetemplate, this implies that the shift from the origin of the messageimage can be derived by circularly correlating 44 |M′(x−Δx,y−Δy)| 42with T(x,y) 40, since:|M′(x−Δx,y−Δy)|{circle around (×)}T(x,y)=δ(x−Δx,y−Δy).  (6)

Therefore, the result of the correlation will be a n×n correlagram image46, whose highest peak will be located at the desired shift distance,(Δx,Δy). This peak location can be found 48 and used to compute theshift (Δx,Δy). The shift is then applied to align 50 the extractedmessage image, which allows for the correct interpretation of theembedded message bits.

In the present invention, the offset of the tiles between consecutiveframes can be deterministic or random. A deterministic offset has theadvantage that once the spatial shift is known for one frame, thespatial shift for the other frames can be easily computed. For adeterministic offset, one could use a state-transition table, where thex or y offset value in the current frame (i.e., the current state) isdetermined by the x or y offset value from the previous frame (i.e., theprevious state). After a specified number of frames, the current statereturns to the initial state. An even simpler method is to add aconstant x or y offset to the previous x or y offset value. However, arandom offset may help to further reduce the visibility of thewatermark.

A random offset for each frame can be produced by a variety of differentapproaches. In general, we need to generate a pair of random numbersthat can be mapped to a (x,y) pair of integer pixel displacements. Thismapping can be a simple 1-to-1 mapping. One approach is to derive therandom numbers from a PN sequence. For simplicity, the seed value (key)could be the same as that used in generating the carrier for thewatermarking process, but a different key and/or a different randomnumber generation process could also be used. Another approach is to usesome unique attribute of each frame of the image sequence in the randomnumber generation process. Such attributes include, but are not limitedto, the frame number or a time stamp. By representing the attribute as am-bit number, it can then be used as the seed value for the PN sequencegeneration. It is also possible to apply a hashing function directly tothe m-bit value to derive a n-bit value (n<m), where the n-bit value isthe integer pixel displacement. Different random number generators ordifferent hashing functions can be used to derive the (x,y) offset pairfrom the same m-bit attribute value.

Since the same carrier image is used for each frame, the tiles from anynumber of frames can be combined after determining the offset of eachframe. This improves the constructive addition of the dispersed messageimage. Moreover, the summation of the tiles from multiple frames willresult in improved destruction of the original image content, becausethe content often varies significantly over a number of frames. Even ifthe original image content is static between frames, the differentoffsets of the tiles insures that different content is used in eachframe. These properties increase the robustness of the watermark byincreasing the signal-to-noise ratio of the extracted message image,which provides improved protection against certain removal attacksand/or allows for the amplitude of the watermark to be reduced to alower level. Reducing the amplitude further reduces the visibility ofthe watermark.

In some applications of the present invention, it may be desirable touse the same spatial shift in several consecutive frames, rather thanchanging the spatial shift with each frame. This may provide additionalrobustness to the watermark extraction process when the image sequencedata has been modified during certain types of attacks. For example, ifa video camcorder is used to capture an illegal copy of a projectedmovie in a theater, there is a mismatch of the temporal sampling ratesof the projected image (24 progressive frames per second) and the videocamcorder (60 interlaced fields per second). If the offsets are changedwith each frame, there will be occasions when the camcorder willintegrate different watermark patterns over two frames. By allowing thesame watermark pattern to persist for two frames, there is an increasedprobability that the watermark can be extracted from any field orinterlaced frame of the illegal video copy. Of course, increasing thedisplay duration of a watermark pattern with the same offset beyond twoframes might further increase the robustness of the extraction process,but the slowly changing watermark pattern will also be more easilyperceived than one that is changing every frame or every other frame.

It is worthwhile to note that the circular shifting of the tile patternis entirely equivalent to circularly shifting either the carrier imageor the message image in the preferred embodiment. This is a result ofthe circular convolution that is used when creating the dispersedmessage image. For a given implementation of the present invention, itmay be advantageous to perform the spatial shifting of the watermarkpattern using either a circular shift of the tile pattern, a circularshift of the carrier image, or a circular shift of the message image.

Another benefit of the invention is that a variable offset betweenframes also provides the opportunity to embed additional information inthe image sequence. By considering a sequence of offsets (or offsetdifferences) associated with a group of consecutive frames, we can embedand then extract additional message data. This data could be related tothe message data that is embedded in each individual frame, or it couldbe completely different information such as a time stamp associated withthe group of frames. As an example, consider a simple scheme where wewish to embed N bits of information (a presentation time stamp, forexample) over a group of N consecutive frames in the original sequence.We can then associate one bit with each frame by the following process.If the offset for a frame is less than a pre-specified threshold, thecorresponding bit is a ‘0’, and if the offset is greater than thethreshold, the corresponding bit is a ‘1’. It is worth noting that thisprocess of embedding information using the offset of the tiles can bealso applied to an image sequence watermarking method that usesdifferent keys or different message data for each frame. However, asdescribed previously, these methods still suffer from limitations ascompared to the present invention.

While the invention has been discussed in terms of the spatial domainwatermarking process as described by Honsinger et al., it is obvious howthe same method can be applied to any spatial domain watermarkingprocess that allows the watermark pattern to be shifted during theembedding process and subsequently synchronized during the extractionprocess. The invention can also be used for some types of frequencydomain watermarking methods. In particular, many frequency domainwatermarking methods use block-based transforms such as the 8×8 DCT thatis used in JPEG and MPEG compression systems. Some methods apply thewatermark directly to the compressed bit stream (such as the methoddescribed by Girod et al.), and the present invention cannot be appliedto these methods because the DCT block locations are fixed. However,other frequency domain methods use the DCT outside of a compressionframework, and these methods can easily shift the DCT block locationsfrom frame to frame.

For completeness, we note that correction for rotation, scaling(magnification), and skew is another fundamental element of all robustdata embedding techniques. For shifted tiles to be synchronizedproperly, it may be necessary to first correct for rotation, scale, andskew. In Honsinger, et. al., U.S. Pat. No. 5,835,639, “Method fordetecting rotation and magnification in images”, a preferred method ofcorrection of rotation and scale is described. The correction techniquerelies on autocorrelation of the watermarked image. For example, for awatermarked image that has not been rotated or scaled, we would expectto see autocorrelation peaks spaced horizontally and vertically atintervals of n pixels and n lines, where this spacing is related to then×n tile structure of the dispersed message image. At the zero offsetcorrelation point, there is a very high peak due to the imagecorrelating with itself. Now, if the watermarked image is scaled, thepeaks must scale proportionately. Similarly, if the watermarked image isrotated, the peaks must rotate by the same amount. Therefore, therotation and scale of an image can be deduced by locating theautocorrelation peaks. Importantly, because autocorrelating thewatermarked image requires no extra calibration signal, it does not taxthe information capacity of the embedding system. In addition, thistechnique can be applied to any embedding technique with redundantembedded signals and may implemented on a local level to confront loworder geometric warps.

Because the watermarking process as described by Honsinger et al. isrobust to rotation, scale, and skew, it is possible for the watermarkpattern to be rotated, scaled, or skewed from frame to frame, ratherthan shifted as is done in the present invention. These operations mayalso reduce the visibility of an embedded watermark in a sequence, butthey are not preferred over shifting for several reasons. First, thelocal changes in the watermark pattern from frame to frame when usingrotation, scale, or skew are not as substantial as those that can beobtained by shifting. For example, rotation can provide significantchanges away from the center of rotation, but there will only be verysmall local changes around the center of rotation. Of course, theseoperations could be combined with shifting to produce even greater localchanges than would be obtained using only one method. Second, thedetermination of rotation, scale, and skew when extracting the watermarkis a more computationally intensive process than the determination ofthe shift. Likewise, changing the watermark pattern using scale,rotation, and skew during the embedding process requires morecomputations than simply shifting the tiles (or equivalently, shiftingthe carrier or message as described previously). Finally, the use ofrotation, scale, and skew for changing the watermark pattern does notallow the information from multiple frames to be easily combined. Withshifting, it is a simple matter of translating the tiles to a commonorigin, while the other methods require affine transformations that aremore computationally demanding.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   10 two dimensional original image-   12 watermarked image-   12′ contiguous tiles from watermarked image-   14 two-dimensional message image-   14′ extracted message image-   16 message icon-   18 message bits-   20 circular convolution image step-   22 carrier image-   24 dispersed message image-   26 secure key-   28 scale image step-   30 scaled dispersed message image-   32 average of individual tiles step-   34 averaged tile-   36 circular correlation step-   40 message template T(x,y)-   42 shifted extracted image |M′(x−Δx,y−Δy)|-   44 circular correlation step-   46 correlagram image-   48 peak location step-   50 aligning extracted message image step

1. A method for embedding message data in an image sequence having twoor more frames, comprising the steps of: a) providing a dispersedmessage image representative of the message data; b) producing acyclically shifted version of the dispersed message image correspondingto a spatial offset; c) combining the cyclically shifted version of thedispersed message image with a frame of the image sequence; and d)repeating steps b) and c) for one or more additional frames of the imagesequence.
 2. The method claimed in claim 1, wherein the step ofproducing a cyclically shifted version of the dispersed message imageincludes the steps of: a1) producing a message image representing themessage data; a2) providing a carrier image; a3) convolving the messageimage with the carrier image to produce a dispersed message image; anda4) cyclically shifting the dispersed message image by a spatial offsetto produce a cyclically shifted version of the dispersed message image.3. The method claimed in claim 1, wherein the cyclically shifteddispersed message images are not visible when added to the frames of theimage sequence.
 4. The method claimed in claim 2, wherein the carrierimage has random phase and substantially flat Fourier amplitude.
 5. Themethod claimed in claim 1 wherein the cyclically shifted versions of thedispersed message image are shifted randomly for successive frames. 6.The method claimed in claim 1, further comprising the steps of: e)determining the spatial shift corresponding to each cyclically shiftedversion of the dispersed message image in a plurality of frames; f)aligning the plurality of frames based on the determined spatial shiftscorresponding to the respective dispersed message images and combiningthe aligned frames to produce a combined frame; and g) extracting themessage image from the combined frame.
 7. The method claimed in claim 2,further comprising the steps of: e) determining the spatial shiftcorresponding to each cyclically shifted version of the dispersedmessage image in a plurality of frames; f) aligning the plurality offrames based on the determined spatial shifts corresponding to therespective dispersed message images and combining the aligned frames toproduce a combined frame; and g) extracting the message image from thecombined frame by correlating the carrier image with the combined frame.8. A system for embedding message data in an image sequence having twoor more frames, comprising: a) means for providing a dispersed messageimage representative of the message data; b) means for producing acyclically shifted version of the dispersed message image correspondingto a spatial offset; c) means for combining the cyclically shiftedversion of the dispersed message image with a frame of the imagesequence; and d) means for repeating initiating repetition of the meansfor producing and means for combining.
 9. The system claimed in claim 8,wherein the means for producing a cyclically shifted version of thedispersed message data image includes: a1) means for producing a messageimage representing the message data; a2) means for providing a carrierimage; a3) means for convolving the message image with the carrier imageto produce the dispersed message image; and a4) means for cyclicallyshifting the dispersed message image by a spatial offset to produce acyclically shifted version of the dispersed message image.
 10. Thesystem claimed in claim 8, wherein the cyclically shifted versions ofthe dispersed message image are not visible when added to the frames ofthe image sequence.
 11. The system claimed in claim 9, wherein thecarrier image has random phase and substantially flat Fourier amplitude.12. The system claimed in claim 8 wherein the means for cyclicallyshifting the dispersed message image employs random spatial shifts. 13.The system claimed in claim 8, further comprises: e) means fordetermining the spatial shift corresponding to each cyclically shiftedversion of the dispersed message image in a plurality of frames; f)means for aligning the plurality of frames based on the determined shiftapplied spatial shifts corresponding to the respective dispersed messageimages and combining the aligned frames to produce a combined frame; andg) means for extracting the message image from the combined frame. 14.The system claimed in claim 9, further comprises: e) means fordetermining the spatial shift corresponding to each cyclically shiftedversion of the dispersed message image in a plurality of frames; f)means for aligning the plurality of frames based on the determinedspatial shifts corresponding to the respective dispersed message imagesand combining the aligned frames to produce a combined frame; and g)means for extracting the message image from the combined frame bycorrelating the carrier image with the combined frame.
 15. The methodclaimed in claim 1, wherein the step of producing a cyclically shiftedversion of the dispersed message image includes the steps of: a1)producing a message image representing the message data; a2) cyclicallyshifting the message image by a spatial offset to produce a cyclicallyshifted message image; a3) providing a carrier image; and a4) convolvingthe cyclically shifted message image with the carrier image to produce acyclically shifted version of the dispersed message image.
 16. Themethod claimed in claim 1, wherein the step of producing a cyclicallyshifted version of the dispersed message image includes the steps of:a1) producing a message image representing the message data; a2)providing a carrier image; a3) cyclically shifting the carrier image bya spatial offset to produce a cyclically shifted carrier image; and a4)convolving the message image with the cyclically shifted carrier imageto produce a cyclically shifted version of the dispersed message image.